With the growing interest in energy conservation, increasingly more machines, such as mobile industrial work machines or stationary power generation machines, are supplied with electric drive assemblies or systems for operating various tools or functions of the machine. Ongoing developments in electric drives have made it possible for electrically driven machines to effectively match or surpass the performance of mechanically driven machines while requiring significantly less fuel and overall energy. As electric drives become increasingly more commonplace with respect to such machines, the demand for more efficient generators and techniques for controlling same has also increased.
Among the various types of electrically driven machines available for use with such electric drives, switched reluctance (SR) machines have received great interest for being robust, cost-effective, and overall, more efficient. An SR machine is typically used to convert mechanical power received from a primary power source, such as a combustion engine, into electrical power for performing one or more operations of the machine. Additionally, an SR machine may be used to convert electrical power stored within a common bus or storage device into mechanical power. SR machines can similarly be used in conjunction with other generic power sources, such as batteries, fuel cells, and the like. Still further, SR machines can also be used with stationary machines having conventional power sources such as windmills, hydro-electric dams, or any other generic power source commonly used for stationary applications.
A typical SR machine essentially includes a multi-phase stator that is electrically coupled to an electric drive circuit, and a rotor that is rotatably positioned within the stator. In a motoring mode of operation, the electric drive selectively enables gates or switches associated with each phase of the stator so as to cause electromagnetic interactions between the stator and rotor poles and rotate the rotor relative to the stator at a desired torque and/or speed. Alternatively, in a generating mode of operation, the electric drive may be configured to receive any electrical power which may be induced by mechanical rotations of the rotor relative to the stator. The electric drive may use the electrical power that is induced during the generating mode to power auxiliary or accessory devices of the associated work machine, or in some cases, store the electrical power in an energy storage device.
Conventional schemes for controlling SR machines may involve operating two switches associated with each phase of the stator, or current chopping, in one of a number of different operating modes. For instance, control for operating modes corresponding to a first range of speed tasks may be conducted by hard chopping current to the two switches of each phase, while control for a second range of speed tasks may be conducted by soft chopping current to the two switches of each phase. A conventional hard chopping routine sources a pulsed phase current by simultaneously opening and closing both switches of each phase at the required switching frequency, whereas a conventional soft chopping routine sources soft pulsed phase current by holding one of the switches closed while opening and closing the other switch at the switching frequency.
Although functional, conventional SR machines offer significant room for improvements in terms performance and efficiency. One commonly shared area of interest relates to improving the accuracy of torque production, or maintaining the average torque output of a machine, such as an SR machine, at a more consistent level. The average torque output may be better managed by improving the switching strategy or chopping control scheme being applied per phase of the SR machine. However, adjustments in the chopping scheme are limited due to various hardware constraints. Among other things, the switching frequency as well as the turn-off point for each phase cannot be adjusted without adversely affecting machine components, thus leaving only the turn-on point of each phase as a point of adjustment.
The turn-on and turn-off points of each phase may be managed by the respective bounds or limits of a predefined hysteresis band. These limits of the hysteresis band may further be preconfigured according to any one of a number of different techniques. For example, some techniques establish a generally wider hysteresis band to accommodate for a wider range of phase current fluctuations, while some other techniques adjust hysteresis bands based on rotor position by affecting machine inductance. However, use of such techniques often results in higher currents, which further leads to increased losses in power, higher operational temperatures and increased risks of over-current conditions.
In order to help prevent such increases in current, other techniques have also been used which engage only the turn-on point, or the lower limit of the hysteresis band, to be adjusted and lowered. However, due to machine inductance, current rise rates, and in some instances, current fall rates, the lower limit of the hysteresis band can exhibit a drooping effect. A drooping effect may be caused by decreases in machine inductance, which cause the current to rise at faster rates. Moreover, as the current rises at faster rates, more time is required to allow the current to sufficiently fall in order to satisfy the switching frequency of the machine. In response, the average observed phase current gradually decreases, or droops, and increasingly departs from the initially desired or target phase current. Such decreases in the average phase current further lead to lower average torque production and an overall reduction in torque accuracy.
Accordingly, there is a general need for improved hysteresis-based controls which perform with more consistency and accuracy while satisfying various hardware constraints. In one particular instance, there is a need for an improved hysteresis-based control for use with current chopping in SR machines which overcomes the deficiencies identified above. Specifically, there is a need to improve the accuracy of the torque produced by an SR machine, and more consistently maintain a desired average torque output thereof. Moreover, there is a need to better maintain the average phase current of the machine at a consistent level while staying within the limits of the machine's switching frequency without introducing high magnitudes of current.